Recognize that `TempFile` is a file.

The problem was that, since `wkhtmltoimage` [does not work with HTML in STDIN](http://code.google.com/p/wkhtmltopdf/issues/detail?id=534),
the HTML must be stored in a temporary file, and the original IMGKit thinks that
[Tempfile instances are not files](https://github.com/csquared/IMGKit/blob/master/lib/imgkit/source.rb#L14).
Merge pull request #46 from topspin/recognize-tempfiles

Recognize that `TempFile` is a file.